Episode 21 New Tiller

I broke my old tiller, by leaning on it. I'm sure I have a knack for finding the weakness in any thing or system the first time I touch it, leading to its immediate breakage. I've just got to think about how to market this talent; something like "Need to test your thing or system? just give it to me and I'll find the first thing that breaks in record time".

Actually, the old tiller came with the boat, and has survived my attentions until now. It split along the grain at the back, where it pivots on a bolt. This area is also the most exposed to the weather, so it is not surprising that it split there. When I broke it, I was about 5 nautical miles from home, and had to nurse the boat back by holding the tiller in the rudder stock. It was not a disaster, but it was awkward when I needed to hook up Otto*, the ST1000 Tiller Pilot.

(* Stolen from the 2008 movie WALL-E**)

(** Its a pun, Otto/AUTO/Autopilot***)

(*** Bad dad joke)

So, I decided to make a new tiller.

The first thing I needed was a suitable piece of wood. Pine would be fine; light and easy to work, but I was open to suggestions.

I went to a local specialist timber merchant, nearby. "Timber", it said proudly over the gate. It also sported a very large shed full of nicely cut arboreal material of different types, sizes and lengths. I handed the old tiller to the fellow over the counter and asked, hopefully, whether he could sell me something to replace it with. He scratched his chin, and told me that he could not. Pine, apparently, was not sustainable. What about another timber? No. Too heavy and difficult to work. Perhaps I should try Louis' Woodturning on the other side of town.

I might have missed something here, but directing me to another retailer (which happened to be closed at the time) did not strike me as a sustainable business strategy.

Finding myself in the position of John Cleese, upon entering Henry Wensleydale's Purveyors of Fine Cheese, I decided that arguing was pointless, and left with my tiller and wallet fully in tact.

Fortuitously, I found a nice piece of pine in my little hoard in the back yard. It was an off-cut from the bunks I had made for the trailer. Further, it had been CCA treated for outdoor use, so promised to be quite durable. I cut out the billet, and began to shape the handle.

Billet for new tiller, with old tiller at rear

Shaping the handle end

Knowing that the old tiller had split at its pivot-point, I reinforced the rudder-stock end with a couple of pieces of aluminium angle. The aluminium also served as packers, to take out the slack between the timber tiller and alloy rudder-stock, and spread the loading from the rudder stock onto the soft timber so that it would not get crushed.

I needed to cut some grooves into the aluminium angle, because the tiller stock had two cleats attached for securing the rudder in the up or down position. These cleats were held by round-headed bolts, with the round-heads protruding into the rudder stock. The old tiller accommodated these bolt-heads with some roughly gouged grooves. It took a little adjustment to get a snug fit. I shaved off the back a little, to allow the tiller to rotate freely on its pivot, and cut the length down to avoid a clash with the main sheet. The new tiller is longer than the old, which allows me to stretch a little less, when reaching for something forward with my free hand.

New tiller with aluminium angle reinforcement at rudder-stock,  ST1000 bracket fixed, and cover, with old tiller behind.
Another small improvement was the shape of the handle. The old tiller had a rounded square at the handle, but my version had a kind of upside down fat teardrop, which is easier on the hands and legs (one technique I am developing is to get both hands free by draping a leg over the tiller, and steering with that).

The new tiller now needed some external varnish. I think I have finally figured out why my previous attempts at varnishing have been rubbish. The missing ingredient was patience. 

What I had been doing was applying wet varnish to all four sides of something, including the new tiller. This inevitably led to drips and runs of gloopy stuff that would not harden, especially on the undersides. I think the drips carry the more gelatinous fractions of the varnish compound with them, so you end up with a soft, gloopy snot that you can scrape off with your fingernail. Its probably obvious to experienced varnishers, but is seldom mentioned in on-line tutorials

When I restricted myself to applying the wet varnish to the upper, flat surfaces only, I found that the gloopy snots would not form, and the coat would smooth out and harden within 4 to 8 hours, meaning that I could apply two every day. This turned out to be much faster than waiting a week or so for the gloopy snots to harden, and then attempting to sand them down. I could then lightly sand the hardened coat, and apply another layer, per the tutorials. 

My other tip is that "lightly" sanded means very little downward pressure on the sanding paper. You should apply just enough pressure to move the sandpaper back and forth. Too much pressure, and you could rip the varnish off in rolls or crumbs. However, I found that to be less of a risk when there were none of those gloopy snots to deal with. This shows you the importance of only applying the wet varnish to flat, horizontal surfaces, so that the gloopy snots don't form.

Finally, I thought about getting a cover for the tiller, to keep the sun of my shiny varnish. The old tiller had crazed and flaked to a grey mess in the sun, and I liked the look of the honey-coloured pine grain. It occurred to me that, in my hoard of stuff that may one day be useful, I had the "sock" that an old hammock had come in. To my surprise, it was the perfect length and diameter. 

In the end, the only new materials in my new tiller were the aluminium brackets and a tin of external varnish. The rest came from the hoards in my back yard. That's not to justify hoarding as a good thing in general, but just saying ...
Video of new tiller in the capable hands of Otto the TillerPilot

Episode 20 Theoretical Hull Speed

I had thought about calling this blog "How fast is my boat", then recalled that we're talking sailing here and a more appropriate title would be "How slow is my boat". Then I split the difference. Kind of.

According to boat-lore, longer boats are faster. You'll find formulae on the interweb that will work out the theoretical hull speed, but not many explanations about how the formulae were derived. So, I'll give it a go, based on my understanding of hydraulics.

The formulae are based on wave theory, but what has that got to do with boats? Firstly, sailing boats are displacement boats, meaning that they sit in the water, rather than skimming over the top of it like planing boats (or the ultra-modern sailing boats with foils). Displacement boats have to push the water out of the way to move forward. Most importantly, a displacement boat will sit in the trough of a wave, with the front peak at the bow, and the rear peak somewhere behind. The front peak is always at the bow, but the position of the rear peak determines the magical number - the theoretical hull speed.

The other important fact to bear in mind is that the speed of the wave is a function of the distance between the peaks (the wavelength). So, the faster the boat goes, the longer its wave becomes, until the rear wave peak falls behind the stern of the boat.

Try to picture three cases in your mind, looking at a boat from the side as it sails past;

  1. Front wave peak at the bow, rear wave peak half way along the boat. Both the front and rear of the boat are supported by the wave peaks, so the boat is sailing on the flat, lengthwise. The boat is simply overcoming friction.
  2. Front wave peak at the bow, rear wave peak at the stern. Again, both the front and rear of the boat are supported by the wave peaks, so the boat is still sailing on the flat, lengthwise.The boat is simply overcoming friction, again.
  3. Front wave peak at the bow, rear wave somewhere behind the stern. Things now change because the stern is not supported, so it drops. From the point of view of the boat, it now needs to climb uphill to keep up with the front wave peak. The boat needs to expend extra energy to climb the up the trailing face of the wave. Not only is the boat overcoming friction, it is climbing against its own weight, which tends to push if back, away from the front wave peak, thus slowing it down.

The additional energy needed to climb the trailing face of the wave becomes a case of diminishing returns, the faster we go, the longer the wave, and the steeper the climb up the trailing face of the front wave peak. The theoretical hull speed, then, is not a fixed limit, but it becomes harder to overcome both friction and the climb up the wave as the speed of the boat increases.

That's why it is possible to drive a sailing boat beyond its theoretical hull speed. You can do it with a motor boat, which will push it all the way to the top of the front wave peak but, then, you'll be planing.

And so, we can get to the formulae, which are a mix of theory and empirical observation. A useful one relates theoretical hull speed to the square root of the length of the water line

V = 1.34 x √LWL
Where

V = Theoretical hull speed in Knots
LWL = Length of water line in Feet

Note that this relates to LWL (length of waterline), not LOA (length overall).

Applying this to an eclectic selection of boats yields the following table. Apart from the last two boats, I have not included boats above 32 foot. This is because they become much more expensive above 30-35 feet, and they will likely need crew. Bigger might be better, but there is a trade-off in dollars and the ability to sail off on your own.

Boat LOA (ft) LOA (m) LWL (ft) LWL (m) Theoretical hull speed (knots)
Cygnet 20 19.3 5.9 17.7 5.4 5.6
Austral 20 20.0 6.1 17.0 5.2 5.5
RL 24 24.0 7.3 19.6 6.0 5.9
Noelex 25 25.5 7.8 22.1 6.8 6.3
Ross 780 25.6 7.8 23.3 7.1 6.5
Austral Clubman 8 26.7 8.2 25.3 7.7 6.7
RL28 28.1 8.6 23.6 7.2 6.5
Hanse 315 31.6 9.6 28.5 8.7 7.2
Cavalier 32 32.0 9.8 24.0 7.3 6.6
Suhaili 44.0 13.4 28.4 8.7 7.1
Ranger (J Class) 135.0 41.1 87.0 26.5 12.5

This table indicates that my Austral 20 has a theoretical hull speed of 5.5 knots. However I have pushed it to 6.5 knots and just touched 7.5 knots briefly, with a crew, coming down off a wave. It depends on which point you're sailing, how many spare hands you have, and the state of the sea. For instance, if I'm closed hauled and climbing upwind, I'd be happy with 3.0 knots, but if I bear away on a broad reach in the same conditions, I could do 6.5 knots. In the light of these experiences, I look at the theoretical hull speed as an indicator of the average upper speed of your boat. My Austral 20 might do a circuit of Peel Island at an average of 5.5 knots, but it would be prudent to allow for 5.0 knots, or even 4.0 when planning the day's sailing. If I were to upsize to an Austral Clubman 8 or Cavalier 32, I might be able to increase these allowances to 6.0 or 5.0 knots. The gain of 1 or 2 knots is not insignificant when you consider that tidal currents in Moreton bay could get up to about 1.5 knots against you.

Compare Sir Robin Knox-Johnson on Suhaili, who was the first person to sail solo around the world non-stop, covering 30,123 Nautical Miles in 313 days. His average speed was 4.02 knots (A World of My Own, Robin Knox-Johnson, 1969), which was much less than the theoretical hull speed of the boat (7.1 knots). However, he did not have the luxury of picking his conditions or scampering to port when the winds were either too light or too fresh.

Finally, it seems the boat manufacturers have done their sums, and know that most new boat-owners are likely to be fair weather sailors. The older designs, after the likes of Suhaili, have a considerable shorter LWL than LOA. The reason is that the boat is more "sea-kindly" with a sloping bow and counter (overhang) at the back.

So, for instance, an older design like the Cavalier 32 has a lot of boat above the water at the bow and stern. This makes it more comfortable in heavy weather. It slams less, when the front of the boat drops off a wave and hits the water below, because the bow is more knife-like. Its the difference between slapping water with the palm of your hand, and doing a karate-chop on it. It is also less likely to get pooped, when a trailing sea dumps a large volume of water in the cockpit, because the approaching wave will pick up the counter before hitting the cockpit. However, these sea-kindly features come at the expense of accommodation. Pinched ends generally mean smaller bunks and cramped cockpits.

The other factor is that although LWL gives you speed, LOA is what you actually pay for. Marinas tend to charge per foot (or metre) of the overall length, per berth. So, the difference between LOA and LWL becomes an "overhead". If you want to reduce the overhead, you need to reduce the difference between LOA and LWL, which brings us to the newer boat designs, such as the Ross 780, Austral Clubman 8 or Hanse 315.

These boats have plumb ends, which give you more theoretical hull speed per foot or metre of LOA. They also have larger accommodations; the Hanse 315 as a stern wide enough to place a double bed width-ways across the boat, which makes good use of the available space. They might be less sea-kindly than the older designs, but the manufacturers probably reckon that their owners would have tied up safely in their marinas than put to sea in rough conditions, anyway.

In conclusion, then, the theoretical hull speed is a guide, rather than a limit. Also, there is a trade off between speed, sea-kindliness and accommodation. Generally, the bigger the better, but bigger boats are more expensive and might be difficult to sail single-handed.

Episode 47 Stove Box Mark 3

Stove Box Mark 1 was large and heavy. I had built it for the Austral 20 because it had no galley. It was made from 12mm ply, lined with ceme...